Cryptographic techniques in information transmission have been studied in response to recent dramatic advance in information/communication technologies such as electronic commerce transactions, electric mails, etc. Quantum cryptography is one of cryptographic techniques, which draw much attention recently.
The quantum cryptography provides security by means of physical phenomenon by the uncertainty principle of Heisenberg in the quantum theory. According to the uncertainty principle, the state of quantum will be changed once it is observed, wiretapping (observation) of communication will be inevitably detectable. This allows to take measures against the wiretapping, such as shutting down the communication upon the detection of wiretapping. Thus, quantum cryptography makes wiretapping impossible physically. Moreover, the uncertainty principle explains that it is impossible to replicate particles.
Quantum teleportation is one of the key features of quantum cryptography. Quantum teleportation is a technique to transfer quantum information of the particles from one place where the particles exist to another place. The quantum teleportation is realized by information communication between photons by utilizing entanglement of quantum (quantum entanglement). Photon pairs in the quantum entangled state have such a property that once a quantum state of one of the photons is determined, then that of the other can be spontaneously determined. This property is not dependent on a distance between the photons.
Incidentally, optical lithography is utilized as a fine fabricating technique for semiconductors today. Lithography is a technique for forming an image based on an image transferred by exposure to light. In the conventional lithography, a limit of spatial resolution due to light diffraction makes it difficult to make fabrication finer than the order of light wavelength. On the other hand, quantum lithography, that is, a technique to attain a finer fabrication than the classical diffraction limit in the conventional technique by utilizing a phenomenon that de Broglie wavelength becomes shorter in a multiphoton in the quantum entangled state, has been proposed and studied. (see “A. N. Boto, P. Kok, D. S. Abrams, S. L. Braunstein, C. P. Williams, and J. P. Dowing, ‘Physical Review Letters’ 85 (2000) p. 2733-2736” and “D. V. Strekalov and J. P. Dowing, ‘J. Mod. Opt’ 49 (2002), p. 519.”).
In the quantum teleportation and quantum lithography, photon pairs in the quantum entangled state are inevitably required.
So far, parametric down-conversion (PDC) by using a nonlinear crystal has been used frequently to generate photon pairs in the quantum entangled state. (See P. G. Kwiat, K. Mattle, H. Weinfurter, and A. eilinger, “Physical Review Letters” 75 (1995) p. 4337-4341), and “P. G. Kwiat, E. Waks, A. G. White, I. Appelbaum, and P. H. Eberhard, “Physical Review A” 60, Number 2 (1999), R773-R776”.
Meanwhile, generation of quantum-entangled photon pairs by using a semiconductor has been proposed recently. Especially, cascade light emission occurring in quantum dots by biexciton is disclosed in “C. Santoni, D. Fattal, M. Pelton, G. G. Solomon, and Y. Yamamoto, “Physical Review B” 66 (2002) 045308”.
Moreover, in “S. Savasta, G. Martino, R. Girlanda, ‘Solid State Communications’ 111 (1999) p. 495-500”, hyper parametric scattering (hyper Raman scattering), which is in resonance with biexciton in a semiconductor crystal, is theoretically pointed out.
However, in the parametric down-conversion, a pair of quantum-entangled photons is generated from a parent photon (pump photon) via second-order non-linear process. Thus, wavelengths of the photons thus generated are double that of the parent photon. Therefore, the parametric down-conversion is difficult to generate photon pairs in a short wavelength range, and inapplicable to generation of the quantum entanglement of multiphotons, quantum lithography, and the like.
Moreover, “C. Santoni, D. Fattal, M. Pelton, G. G. Solomon, and Y. Yamamoto, “Physical Review B” 66 (2002) 045308” discusses that structural asymmetry of quantum dots prevents generation of entangled photon pairs for polarization.
Furthermore, “S. Savasta, G. Martino, R. Girlanda,’ Solid State Communications’ 111 (1999) p. 495-500”, is disclosure of a theory and leaves it uncertain whether the contents thereof can be reduced to practice or not. Moreover, this literature also discusses temporal correlation of the generated photon pairs, but does not disclose the quantum entanglement regarding polarization.
The present invention, which is accomplished in view of the aforementioned problems, has an object of attaining a method for generating quantum-entangled photon pairs, in which each photon is in a short wavelength, and which makes it possible to generate quantum-entangled photon pairs regarding polarization.